Interpretive Summary: Water quality issues in watersheds in the state of Iowa have been a source of concern for many years. Although, there has been extensive monitoring of the various watersheds there have been few efforts to develop a strong analytical approach to determine the meaning of these changes. Fortunately, the Raccoon River has one of the longest periods of records collected among watersheds in the Midwest and we undertook an effort to be able to evaluate the meaning of the trends in nitrate-nitrogen concentrations in this river. We found that there was no significant trend in the nitrate-nitrogen concentrations in the past 17 years and there has been no change in the amount of fertilizer applied to cropland within the watershed. The methods used in this study can be applied to other watersheds because they are more robust in being able to assess changes in nutrient concentrations over time. This information is of value to policy-makers and watershed scientists.

Technical Abstract:
The Raccoon River Watershed in Iowa has received considerable attention in recent past due to frequent detections of nitrogen concentrations above the federal drinking water standard. This paper econometrically investigates the determinants of variation of nitrogen concentrations in the Raccoon River. The analysis relies on a GARCH (1, 1) process to model the serial dependence of volatility of the monthly nitrogen concentrations in the Raccoon River. Monthly nitrogen concentration data from Des Moines Water Works at Van Meter from 1992 to 2008 is used in the study. We find that there is no statistically significant increasing trend in nitrogen concentrations over the study period. There are substantial intra-annual variations in nitrogen concentrations and the existence of a very strong seasonal pattern. Variations in rainfall and temperature contribute more to the monthly variation in nitrogen concentration than do the changes in nitrogen application rates.